Technical Field
The present application relates to recovery of copper from heap leach residues.
Description of the Related Art
Copper sulphide deposits near the surface of the earth react with percolating groundwater. The chemical reactions result in oxidation of the primary copper sulphide minerals, forming a region of secondary copper sulphide minerals, or even a region of copper oxide minerals. These two regions of mineral alteration are known as the “oxide cap” and the “supergene”, both of which sit above the underlying unaltered “hypogene”. The oxide cap is the topmost layer, composed predominantly of copper oxide and copper sulphate minerals. Below the oxide cap is the supergene zone, composed predominantly of secondary copper sulphide minerals, such as chalcocite (Cu2S) and covellite (CuS). The hypogene zone is the bottommost layer, composed predominantly of primary copper sulphide minerals, such as chalcopyrite (CuFeS2) and bornite (Cu5FeS4).
Heap leaching can be used to extract copper from primary and secondary copper sulphide minerals. In heap leaching, ore is stacked in leach pads to a specified height. These ore piles, also known as “heaps”, are irrigated with a solution rich in acid and poor in dissolved copper ions. Chemical reactions within the heap consume acid and release copper ions from the ore, resulting in a solution exiting from the bottom of the heap that is poor in acid and rich in dissolved copper ions. Typically, the acid used is sulphuric acid.
The irrigating solution, which is rich in acid, is typically a raffinate solution from a solvent extraction plant. The solution exiting the bottom of the heap, which is rich in copper ions, is termed pregnant leach solution (PLS). In situations where the ore contains chalcocite (Cu2S) and covellite (CuS), otherwise known as secondary sulphides of copper, the heaped ore is typically aerated from the bottom, as well as irrigated from the top.
While a heap is leached, the ore particles within the heap can break into smaller particles, creating “fines”. Over time, this production of fines results in leached heaps losing their permeability. By the end of a leach cycle, the heap leach residues typically have a particle size distribution which is much smaller than the particle size distribution of the original ore. “Fine particles”, or “fines”, are particles that measure 150 μm or less in diameter, as measured using a wet screen method.
Examination of conventional heap leach data has shown significant size reduction of ore particles as the heap is leached. An example of this is shown in FIG. 1, which shows the particle size distributions of the starting ore and the heap leach residues after 424 days of heap leaching in a heap of 506,000 metric tons. By the end of the heap leach cycle, there were almost twice as much fines in the heap leach residue as in the original ore: 21 weight % vs. 12 weight %.
Bio-oxidative reactions convert the copper in the copper sulphide minerals into extractable copper. The bio-oxidative reactions are performed by bioleaching bacteria. The bioleaching bacteria can modify their environment and establish self-sustaining colonies that maintain the conditions for copper leaching. The bio-oxidative reactions require adequate supplies of oxygen and acid. In order to supply oxygen to the bacteria, it is desirable for the heaps to have free passage of air and percolating leach solution, even though the heaps may be piled several meters high. Segregation and accumulation of the fine particles can reduce free passage of air and percolation of leach solution, thereby reducing the amount of copper leached from the heap.
As with any commercial venture, heap leach operations maximize economic returns by processing highly profitable ores first, and by deferring processing of less valuable portions of the ore body until later. This results in ores with the highest copper grades being mined first, so that the copper grade decreases with time. Regardless of the copper grade, in a typical leaching cycle most of the copper is recovered from the heap at the beginning of the leaching cycle, with less and less copper being extracted towards the end of the leaching cycle. In order to maximize the economic return of the heap, the diminishing returns typically compel the operation to terminate the leaching cycle before all of the available copper is recovered. All of these factors combine to create a situation where the ore being processed at the end of mine life is close in value to the heap leach residues discarded earlier in the life of the mine.
Leaching copper sulphide ore in heaps typically takes several months, with diminishing amounts of copper being extracted as time progresses. On reaching a target extraction value, a heap is taken off-line and allowed to drain down. The resulting heap leach residues are then decommissioned since it is not financially worthwhile to continue to extract the smaller amount of copper still present in the heap leach residues.
Decommissioning heap leach residues can be performed by moving the residues from the pad to a waste dump, or capping the heap leach residues with an impermeable layer. In either case, the heap leach residues, with the remaining copper value, are typically abandoned even if not totally exhausted as a source of copper.
It is possible to reprocess heap leach residues in order to further extract copper.
Chilean Patent No. 1069-02 to Astudillo, M. S. describes a method for turning over part or all of the ore stacked in a heap, at any time during the leach cycle. The action of turning over the ore in a heap restores permeability to air and leach solution, thus improving copper recovery. The method described by Astudillo is essentially an intermission to the primary leach cycle, including an add-on to the end of the cycle, in which the fresh leach residues in the heap are restacked by digging and turning over essentially top to bottom in scoops.
Canadian Patent No. 2,391,091 to Hunter, C. J. describes a method for bacterially assisted heap leaching through the use of a second heap that serves as a bioreactor. This second heap acts to generate bacteria and ferric ion for use in the first heap stacked with ore.
A May 15, 2008 presentation by Garcia, C., Politis, M. and Argandoña, M. in Santiago, Chile at Hydroprocess 2008, International Workshop on Process Hydrometallurgy, entitled “Secondary leaching in Anglo American Chile—Mantos Blancos Division”, describes stacking leached residues from vat leaching in heaps and leaching with sulphuric acid. This allows overall recovery from the original copper oxide ore to increase to 90%, from the initial 78%.
An Oct. 4, 2001 presentation by Yañez, H. in Antofagasta, Chile at I Coloquio de Operadores de Plantas Hidrometalúrgicas de Cobre, entitled “Lixiviación en División Radomiro Tomic”, describes stacking leached residues from heap leaching in dumps and leaching with sulphuric acid. This allows overall recovery from the original copper oxide ore to increase to >80%, from the initial 70%. Oxide ore heap leaching is undertaken without aeration.
It is desirable to provide a method for recovering copper from heap leach residues.